1. Field of the Invention
The present invention concerns fiber optic transmission.
2. Description of the Prior Art
The index profile of optical fibers is generally qualified by the shape of the graph of the function relating the radius of the fiber to the refractive index. The distance r to the center of the fiber is conventionally plotted on the abscissa axis and the difference between the refractive index and that of the fiber cladding on the ordinate axis. The expressions xe2x80x9cstepxe2x80x9d, xe2x80x9ctrapeziumxe2x80x9d and xe2x80x9ctrianglexe2x80x9d are therefore used in referring to the shapes of profiles with graphs which are respectively step-shaped, trapezium-shaped and triangular. The curves are generally representative of the theoretical or set point profile of the fiber and fiber fabrication constraints can result in a significantly different profile.
In modern high bit rate wavelength-division multiplex transmission systems it is advantageous to manage chromatic dispersion, especially for bit rates of 10 Gbit/s and above per channel. The objective is to obtain substantially zero cumulative chromatic dispersion over the link for all values of the wavelength of the multiplex, in order to limit the widening of pulses. A cumulative value of a few hundreds of ps/nm for the dispersion is acceptable. It is also beneficial to avoid zero values of chromatic dispersion for the wavelengths used in the system, for which non-linear effects are greater. Finally, it is also beneficial to limit the chromatic dispersion slope over the range of the multiplex to avoid or limit distortion between the channels of the multiplex.
Stepped index fiber is conventionally used as line fiber in fiber optic transmission systems. For example, the ASMF 200 monomode stepped index fiber has a chromatic dispersion cancellation wavelength xcex0 in the range from 1 300 nm to 1 320 nm and a chromatic dispersion of 3.5 ps/(nm.km) in a range from 1 285 nm to 1 330 nm and 17 ps/(nm.km) at 1 550 nm. The chromatic dispersion slope at 1 550 nm is about 0.056 ps/(nm2.km). The fiber has an effective core area of about 80 xcexcm2.
Dispersion shifted fibers (DSF) have also appeared on the market. In such fibers there is substantially zero chromatic dispersion at the transmission wavelength at which they are used, which is generally different from the 1.3 xcexcm wavelength for which silica has substantially zero dispersion. Thus the non-zero chromatic dispersion of the silica is compensated (hence the use of the term xe2x80x9cshiftedxe2x80x9d) by an increase in the index difference xcex94n between the fiber core and the optical cladding. The index difference shifts the wavelength at which there is zero chromatic dispersion. It is obtained by introducing dopants into the preform during its fabrication, for example by an MCVD process known in the art and not described in detail here.
Non-zero dispersion shifted fibers (NZ-DSF) have non-zero chromatic dispersion at the wavelengths at which they are used. Such fibers have a low chromatic dispersion at these wavelengths, typically less than 6 ps/(nm.km) at 1 550 nm.
One example of such fibers is disclosed in French patent application 98 12431 filed Oct. 5, 1998, whose title in translation is xe2x80x9cLarge effective core area shifted dispersion monomode optical fiberxe2x80x9d and which describes a fiber having a large effective core area, typically greater than 100 xcexcm2. The chromatic dispersion of the fiber in the above application at 1 550 nm is about 8 ps/(nm.km).
It is known that non-linear effects decrease as the effective core area of the fiber increases. In xe2x80x9cA new design for dispersion shifted fiber with an effective core area larger than 100 xcexcm2 and good bending characteristicsxe2x80x9d, ThK1, OFC""98 Technical Digest, M. Kato et al. explain that non-linear effects in the fibers could become the dominant limitations on capacity and transmission distance for long-haul high-capacity amplified transmission systems. The above document indicates that one possible solution is to increase the effective core area of the fibers, to obtain a higher power and a greater interval between repeaters. The above document proposes a fiber having a coaxial profile, surrounded by a pedestal, with an effective core area of 146 xcexcm2 and a chromatic dispersion cancellation wavelength xcex0 of 1 500 nm. The chromatic dispersion at 1 550 nm is low and the chromatic dispersion slope at this wavelength is 0.09 ps/(nm2.km).
U.S. Pat. No. 5,675,690 proposes a monomode optical fiber whose core has a central part with an index lower than that of the cladding, a ring having an index higher than that of the cladding, and a composite ring having a part with an index lower than that of the cladding and a part with an index higher than that of the cladding. In the above document the effective core area is about 85 xcexcm2 and chromatic dispersion is cancelled at a wavelength of about 1 550 nm. The fiber is a DSF in the sense defined above.
The invention proposes an optical fiber which has a large effective core area combined with chromatic dispersion comparable to that of prior art stepped index fibers. In this way the invention provides a fiber which can transmit high-power signals and in which non-linear effects are limited.
To be more precise, the invention proposes an optical fiber having, at a wavelength of 1 550 nm:
an effective core area not less than 100 xcexcm2,
a chromatic dispersion not less than 14 ps/(nm.km), and
a sensitivity to microcurvatures not greater than 1.
The fiber advantageously has a chromatic dispersion at a wavelength of 1 550 nm of not more than 21 ps/(nm.km).
The fiber can have an absolute chromatic dispersion slope at a wavelength of 1 550 nm not greater than 0.07 ps/(nm2.km).
In one embodiment of the invention the fiber has curvature losses at a wavelength of 1 550 nm not greater than 0.05 dB and preferably less than 5xc3x9710xe2x88x924 dB.
In one embodiment of the invention the fiber has a coaxial+buried part index profile.
In another embodiment of the invention the fiber has a coaxial+ring index profile.
In another embodiment of the invention the fiber has a stepped+ring index profile.
The invention also proposes a wavelength-division multiplex fiber optic transmission system including fiber as defined above as line fiber. The system advantageously further includes dispersion compensating fiber.
Other features and advantages of the invention will become apparent on reading the following description of embodiments of the invention which is given by way of example only and with reference to the accompanying drawings.